Well it looks like after our little chat about cleaning the fuel filters and tank on the mog my theory wasn't quite so good after all. Apparently the drain plug is a pig to remove, but me being me started out all confident and was going to get it sorted... How hard can it be right? Well it turns out quite hard actually - the last owner rounded it off trying so I've just left it! I didn't even try!

I also found that the pre filter wasn't a pre filter, it was actually a little pot of crud with some gauze hidden in the middle of it... So that's now clean. The first felt filter was in a poor state too so that's been scraped off (I kid you not, it looked more like a chocolate brownie than a filter!) and then washed. The second was just very dirty but again all cleaned out and good to go. Both went back to white after quite a lot of effort but were still fluffy and dense so not needing replacing.
I had wondered why she pulled well at low revs but ran out of puff when higher up the range. This pretty much explains it - sucking diesel through a pipe full of mud probably isn't the easiest task! I've bled the system and started her up and I've managed to do the job properly. She didn't die after running for some time so I think it's all good.
Next issue is the brakes. I need to bleed them but can't get the bleed nipples on the master cylinder open. And I need to take the wheel off to access the brake cylinder bleed nipples, but I don't have a jack big enough! Grrr!